I am using the 5th gen and it's just about the perfect Linux laptop but a while ago I read that the 6th gen needs some tweaks under Linux, https://delta-xi.net/#056 or https://wiki.archlinux.org/index.php/Lenovo_ThinkPad_X1_Carb... for example.

This may have changed since but the 5th gen is probably a safer choice for the less adventurous

Aye. My X1C1 is still going strong; buying highest spec has always worked well for me. When it says bye, it'll be another one (top model/current gen).